1. Field of the Invention
The present invention relates to an inner-focusing telephoto lens system.
2. Description of the Related Art
In a conventional inner-focusing telephoto lens system for a medium-format camera (6xc3x977; 6xc3x974.5) with a focal length of about 200 mm (equivalent to a focal length of about 135 mm for the 35 mm camera), despite the F-number being relatively larger (e.g., about F4), a large number of lens element, such as seven lens elements, is required, which is disadvantageous for miniaturization of the lens system and the production costs thereof as well.
It is an object of the present invention to provide an inner-focusing telephoto lens system which is constituted by a smaller number of lens elements, and which corrects aberrations sufficiently.
In order to achieve the above object, there is provided a telephoto lens system including a positive first lens group, a negative second lens group, and a positive third lens group, in this order from the object; and focusing is performed by moving the second lens group along the optical axis. The telephoto lens system satisfies the following conditions:
1.0 less than f/f1 less than 1.4xe2x80x83xe2x80x83(1)
1.0 less than |f/f2| less than 1.6xe2x80x83xe2x80x83(2)
wherein
f designates the focal length of the entire lens system;
f1 designates the focal length of the first lens group; and
f2 designates the focal length of the second lens group.
In the above-described telephoto lens system, for the purpose of reducing the number of lens elements, the lens system preferably includes six lens elements in total, e.g., the first lens group includes two positive lens elements and a negative lens element, the second lens group includes a cemented sub-lens group having a positive lens element and a negative lens element, and the third lens group includes a positive lens element, in this order from the object. According to this arrangement, conditions (1) and (2) can easily be satisfied, and thereby the correcting of aberrations can suitably be made.
On the other hand, in the above-described telephoto lens system, if an attempt is made to obtain a much shorter camera-to-object distance (hereinafter, a photographing distance), the traveling distance of the second lens group has to be largely increased, since the power of the second lens group, i.e., the focusing lens group, is determined as defined in condition (2). Therefore in order to reduce fluctuations in chromatic aberration due to the movement of the second lens group, it is preferable to correct chromatic aberration within the second lens group. More concretely, for sufficiently correcting chromatic aberration, the second lens group preferably includes a cemented sub-lens group having a positive lens element and a negative lens element. The order of the positive and negative lens elements is not required. Further, the second lens group preferably satisfies the following conditions:
|n1xe2x88x92n2| less than 0.1xe2x80x83xe2x80x83(3)
0.7 less than {(xcexd1xe2x88x92xcexd2)/(r2xc3x97f2)}xc3x971000 less than 1.1xe2x80x83xe2x80x83(4)
wherein
n1 designates the refractive index of the negative lens element of the cemented sub-lens group;
n2 designates the refractive index of the positive lens element of the cemented sub-lens group;
xcexd1 designates the Abbe number of the negative lens element of the cemented sub-lens group;
xcexd2 designates the Abbe number of the positive lens element of the cemented sub-lens group; and
r2 designates the radius of curvature of the cemented surface of the cemented sub-lens group.
The present disclosure relates to subject matter contained in Japanese Patent Application No.Hei-11-27886 (filed on Feb. 4, 1999) which is expressly incorporated herein by reference in its entirety.